1. Field
The present subject matter relates generally to the field of optical attenuation devices for use in optical systems, particularly to a variable optical attenuator that requires little space and high stability.
2. Description of the Related Art
Fiber optics is increasingly used for transmitting voice and data signals. As a transmission medium, light provides a number of advantages over traditional electrical communication techniques. For example, light signals allow for extremely high transmission rates and very high bandwidth capabilities. Also, light signals are resistant to electromagnetic interference that would otherwise interfere with electrical signals. Light also provides a more secure signal because it does not emanate the type of high frequency components often experienced with conductor-based electrical signals. Light also can be conducted over greater distances without the signal loss typically associated with electrical signals on a copper conductor.
Many conventional electrical networks are being upgraded to optical networks to take advantage of the increased speed and efficiency. Optical communication networks use lasers to create light which is then modulated to convey information. One of the many components of an optical communications network is an optical attenuator. Optical attenuators control the intensity of one or more wavelengths of light within an optical system. To transmit and receive optical light properly without incurring defects or errors, the light intensity needs to be regulated properly. Too much power on the receiver side will saturate the receiver and too little power will result in poor transmission quality. Modern optical networks often use of multiple wavelengths in conjunction with broadband optical amplifiers such as erbium doped fiber amplifies (EDFA), which require proper balancing of the intensity of each wavelengths used in the networks. On occasion, it is necessary to recalibrate or replace one or more of the lasers generating light in the system. To avoid data corruption, it is preferred to completely extinguish the laser's light from the optical system before recalibration or replacement. Optical attenuators are capable of extinguishing the laser's light by blocking it from entering the remainder of the optical system. There are numerous general methods of attenuating or completely extinguishing light including polarization, reflection, diffusion, etc. In addition, it is often necessary to control the intensity of a particular wavelength or channel of light entering a fiber. Although it is possible to simply adjust the electrical current feeding a laser to adjust the output intensity, this is not desirable because this method of attenuation will affect the bandwidth capabilities of the laser. Therefore, it is preferred to use a variable optical attenuator to attenuate or adjust the output intensity of a particular laser.
A variable optical attenuator using crystal wedges is disclosed in U.S. Pat. No. 7,034,979 to Feng et al. (hereinafter, “Feng et al.”). Feng et al. uses crystal wedge as a birefringent element, and one or two polarization modulators between the birefringent element and a reflective element. In Feng et al., the crystal wedge splits an incident beam into two different components depending on their polarizations and refracts them to the one or two polarization modulators. The polarization modulators can be liquid crystal (LC) modules. More specifically, according to Feng et al., in case using a single LC module, as shown in FIG. 1A, the single LC module must convert one polarization of the light to another polarization orthogonal thereto by using LC materials and the structure exhibiting retardation close to quarter-wave plate such as a homogeneously aligned LC module, in which the LC director and optical axis are aligned parallel to the glass plate, because it should be a default-on mode, i.e., normally white mode. In this case, the black or extinction state need to be achieved by reducing birefringence while applying electric field. However, according to the device disclosed in Feng et al., it is very difficult to achieve high extinction due to residual birefringence left in LC module. The dark state transmission of this structure is also dependent on the wavelength of the light due to the residual birefringence. To compensate this shortcoming, Feng et al. has to provide a compensation element on the incident side of the LC module. Moreover, using two LC modules to realize a default-off mode, as shown in FIG. 1B of Feng et al., requires two LC modules with substantially the same thickness to remove all residual birefringence or resulting polarization rotation. It is very cumbersome and quite difficult to match the two cells in exact thickness in practical production. It also makes the overall device size big, which may result in technical disadvantages.
Therefore, there is a need in the industry for a variable optical attenuator that exhibits high extinction with very low wavelength dependence, precise control of incoming light in a compact package. In addition, the optical attenuator is preferably capable of being incorporated into an optical transceiver package.